Fix string casting and remove Flow suppression comments

src/renderers/shared/fiber/ReactChildFiber.js

@@ -117,8 +117,6 @@ function coerceRef(current: Fiber | null, element: ReactElement) {
 
 function throwOnInvalidObjectType(returnFiber : Fiber, newChild : Object) {
   if (returnFiber.type !== 'textarea') {
-    // $FlowFixMe - Intentional cast to string
-    const childrenString = '' + newChild;
     let addendum = '';
     if (__DEV__) {
       addendum =
@@ -136,9 +134,9 @@ function throwOnInvalidObjectType(returnFiber : Fiber, newChild : Object) {
     invariant(
       false,
       'Objects are not valid as a React child (found: %s).%s',
-      childrenString === '[object Object]' ?
+      Object.prototype.toString.call(newChild) === '[object Object]' ?
         'object with keys {' + Object.keys(newChild).join(', ') + '}' :
-        childrenString,
+        newChild,
       addendum
     );
   }

src/renderers/shared/fiber/ReactFiberClassComponent.js

@@ -47,8 +47,7 @@ if (__DEV__) {
       '%s(...): Expected the last optional `callback` argument to be a ' +
       'function. Instead received: %s.',
       callerName,
-      // $FlowFixMe - Intentional cast to string
-      '' + callback
+      callback
     );
   };
 }

src/renderers/shared/fiber/ReactFiberReconciler.js

@@ -168,8 +168,7 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
         callback === null || typeof callback === 'function',
         'render(...): Expected the last optional `callback` argument to be a ' +
         'function. Instead received: %s.',
-        // $FlowFixMe - Intentional cast to string
-        '' + callback
+        callback
       );
     }
     addTopLevelUpdate(current, nextState, callback, priorityLevel);

src/renderers/shared/fiber/ReactFiberUpdateQueue.js

@@ -470,8 +470,7 @@ function commitCallbacks(finishedWork : Fiber, queue : UpdateQueue, context : mi
       typeof callback === 'function',
       'Invalid argument passed as callback. Expected a function. Instead ' +
       'received: %s',
-      // $FlowFixMe - Intentional cast to string
-      '' + callback
+      callback
     );
     callback.call(context);
   }

src/renderers/shared/utils/validateCallback.js

@@ -19,8 +19,7 @@ function validateCallback(callback: ?Function) {
     !callback || typeof callback === 'function',
     'Invalid argument passed as callback. Expected a function. Instead ' +
     'received: %s',
-    // $FlowFixMe - Intentional cast to string
-    '' + callback
+    callback
   );
 }